1. Field of the Invention
This invention relates to a fuel injection control system for internal combustion engines, and more particularly to a fuel injection control system for so-called cylinder injection type (spark ignition type) internal combustion engines in which fuel such as gasoline is directly injected into cylinders through fuel injection valves mounted in heads of the respective cylinders.
2. Prior Art
Conventionally, cylinder injection type internal combustion engines have been developed mainly for use as diesel engines. However, in recent years internal combustion engines of this type have also been intensively studied and developed for use as gasoline engines, as disclosed, e.g. by Japanese Provisional Patent Publications (Kokai) Nos. 62-228642 and 2-291447.
Such cylinder injection type internal combustion engines using gasoline generally employ fuel injection valves and spark plugs both mounted in heads of cylinders of the engine and in which fuel is directly injected into the cylinders through the fuel injection valves and ignited through spark discharge by the spark plugs. Such cylinder injection type engines are free from the fear that fuel is attached to inner walls of the intake pipe, and therefore are advantageous over conventional manifold injection type internal combustion engines in that just a required amount of fuel can be positively supplied into the cylinders, thereby being superior to the latter in respect to fuel consumption, etc.
In cylinder injection type engines, the pressure of fuel supplied to the fuel injection valves (fuel supply pressure) has to be higher than pressure within the cylinders to ensure positive injection of fuel into the cylinders, and therefore also the fuel supply pressure has to be controlled, unlike manifold injection type engines in which fuel is injected into the intake manifold under negative pressure. Specifically, in manifold injection type engines, fuel is supplied into the cylinders through the intake manifold which is placed under negative pressure when each intake valve is open, so that the pressure difference between the fuel supply pressure and absolute pressure within the intake manifold is maintained almost constant, whereby the fuel injection amount can be controlled only by varying the fuel injection period. In contrast, in cylinder injection type engines, fuel is supplied into the cylinders during the compression stroke when pressure within the cylinder assumes a value at least equal to or higher than atmospheric pressure and largely varies as the piston moves within the cylinder. Therefore, it is practically impossible to control the fuel injection amount only by varying the fuel injection period, thus requiring the fuel supply pressure to be controlled in order to determine the fuel injection amount.
However, the pressure within the cylinder (hereinafter referred to as "the cylinder pressure") largely varies with change in the piston position within the cylinder as mentioned above, and accordingly, the required fuel supply pressure also differs depending upon the piston position at which fuel is injected. Particularly, the difference in cylinder pressure between a bottom dead point (BDC) of the piston and a top dead point (TDC) of same is large, making it difficult to accurately control the fuel supply pressure to required values over the entire compression stroke.
Under the above-mentioned circumstances, in conventional cylinder injection type engines in general, when the piston is positioned in the vicinity of the BDC immediately after tje start of the compression stroke at which the cylinder pressure is relatively low (approximately 2-5 kg/cm.sup.2), fuel is injected through the fuel injection valve for supply to the cylinder under a predetermined fuel supply pressure higher than the cylinder pressure, and then the spark plug is caused to effect spark discharge at a piston position close to the TDC when the injected fuel has been evenly dispersed throughout the cylinder space.
However, since conventionally spark ignition is effected in a state where the fuel is evenly dispersed throughout the cylinder space by injecting fuel into the cylinder at a piston position close to the BDC as mentioned above, the exhaust valve is opened when fuel at areas remote from the spark plug is not yet burned, and the unburnt fuel is emitted to the outside as a noxious component. That is, in the conventional engines, during low load operation of the engine hydrocarbons (HC) are emitted, whereas during high load operation of the engine smoke or high concentration NOx is emitted, resulting in degraded exhaust emission characteristics.
To cause firing of fuel, it suffices that a mixture having a predetermined degree of concentration of fuel (i.e., predetermined air-fuel ratio) is present around the spark plug. However, in the conventional cylinder injection type engines, spark ignition is effected in the state where fuel is evenly distributed throughout the cylinder space by injecting fuel at a piston position close to the BDC, i.e. immediately after the start of the compression stroke, which results in an excessive amount of fuel being supplied into the cylinder for firing the fuel, leading to a high rate of fuel consumption.